THE PELVIC LIMB 



That of the former is subtended by a soft pad, and of the latter by a 

 similar pad in addition to a serrated, horny growth which Bailey (1923) 

 affirmed functions like a fine-toothed comb in dressing the fur. 



An additional modification in these as well as all other aquatic mam- 

 mals is the tendency for the elimination of the tubercles or prominences 

 upon the soles of the feet. 



Such slightly aquatic mammals as the marsh and swamp rabbits and 

 water rat {Arvicola), as well as the genus Neofiber, need no further 

 mention in this chapter, for there has been no change in the feet. The 

 feet of the coypu {Myocastor) are moderately webbed. Those of the 

 Australian water rats, Crossojnys, Hydromys and Parahydromys vary 

 from somewhat webbed in the first to unwebbed in the second. 



The aquatic stimuli brought to bear upon the hind feet of all the 

 mammals discussed in the present connection (not Omithorhynchus) 

 seem largely the same or essentially similar. All have tails that are 

 at least moderately long and I believe that without exception all swim 

 by alternate kicks of the hind feet while the fore feet are rarely utilized 

 to any important extent during natation. At one time (Howell, 19'24) 

 I reported catching a glimpse of a water shrew {Neosorex) swimming 

 by kicking the hind feet in unison after the manner of a frog, but I 

 am now of the opinion that I was mistaken, either in identifying the 

 creature so hastily seen or else in my observation, for no one else has 

 ever reported similar action in a small mammal. 



As already partially discussed in a previous chapter, small mammals 

 of this sort quickly abandon any swimming motions of the fore limbs, 

 chiefly, it seems, for the reason that the area of their hands is so much 

 less than that of the feet that not only could they furnish very little 

 propulsive power, but for the same reason they could not act efficiently 

 in neutralizing the asymmetrical action imparted by the hind legs acting 

 alone. For this latter purpose there is an efficient equilibrator already 

 at hand in the tail, which furnishes as much stabilization in the water 

 as does the tail of a kite in the air. Presumably the precise action upon 

 the water of the hind feet kicking in alternation varies in different mam- 

 mals, but it is a point extremely difficult of determination. At any rate 

 there is one uniform result in that as the feet are kicked the effort swings 

 the hinder end of the body from side to side, thus imparting a sinuous 

 motion to the tail. This movement should be involuntary at first, as far 

 as concerns the tail proper, but as the muscles concerned develop with 

 use it is capable of being employed as a definite help to locomotion. 

 Evidence shows that as a result the tail in some manner is modified so 



[279} 



